As disclosed in Japanese utility model application laid-open publications No. 50-154912 and No. 52-92417, there has so far been a joint which is used to connect sprinklers and underground water pipes and to raise the head of sprinklers above the lawn by its double shelled inner and outer pipes engaged with each other by their male and female threads depending on the rise of the ground surface by supplying rich soil to the lawn.
However, in such a joint it is difficult to know how far the head of sprinklers can be raised above the ground surface. On this account, managing to raise its head higher, one sometimes tends to damage the joint threads.
Under the circumstances, the present inventors have made intensive efforts to eliminate the drawbacks and finally accomplished the invention. According to the invention, this kind of conventional joint can be improved by providing a non-threaded portion to the end of female thread on the inside of an outer pipe so that male thread on the outside of an inner pipe can disengage from the female thread when the joint is lengthened up to its maximum length. With this device, those who deal with the sprinkler are able to know their highest position possible and will never damage the threads of the joint.
Also, according to the invention, this kind of conventional joint can be improved by providing a plurality of stepped male threads to the end of the inner pipe with the smallest diametered one at the top and the largest diametered one at the base and providing an annular groove to the foot of each thread. With this device, sprinklers with different diametered female thread can be connected to any one of the male threads on the inner pipe only by cutting off needless male thread thereon at the annular groove. The production cost of sprinklers can thus be reduced by this simple adaptation and the handling becomes simple too.